Over the last one hundred years, global economic activities have increased at least fifty-fold. This extraordinary growth has raised serious concerns about current patterns of both consumption and production. As society has increased its understanding of the environmental implications of its industrial practices, more focus has been placed on the concept of sustainable economic systems that rely on renewable sources of energy and materials. The use of biologically derived polymers is an important component of this relatively new concept of economic development.
Through the transformation of agricultural or marine feedstocks, or the harnessing of enzymes found in nature, a new class of renewable, biodegradable, and biocompatible materials has emerged. Applications of biopolymers range from packaging to industrial chemicals, to computer storage media, and to medical implant devices. In addition to producing environmentally friendly materials with unique physical and chemical properties, the processes used to create biopolymers are receiving attention as possible sources of new manufacturing approaches that minimize energy consumption and waste generation.
In general, biopolymers fall into two principal categories, namely polymers that are produced by biological systems such as microorganisms, plants, and animals; and polymers that are synthesized chemically but are derived from biological starting materials such as sugars, amino acids, natural fats, or oils. Naturally occurring biopolymers include, for example, nucleic acids (DNA and RNA), proteins, polysaccharides (carbohydrates), polyhydroxyalkanoates, polyphenols, polyphosphates and polysulphates. There are several different classes of chemically synthesized biopolymers. Two particular groups include the family of polymers produced from lactic acid, and polymers derived from amino acids.
Whether a process involving biosynthesis, fermentation, methods of recombinant biotechnology, extraction from plants and higher organisms, or the chemical polymerization of naturally occurring monomers, processes for the isolation and production of biopolymer materials generally include procedures for drying the biopolymers. Frequently, the resultant biopolymeric material is provided in powder form and is to be processed by conventional plastic forming techniques such as extrusion and injection moulding.
The drying procedures often involve severe dehydration treatments such as drying with hot air, the use of chemical treatments and/or require lengthy drying periods. It is known, for example, that conventional methods for drying starch with hot air lead to damage of the starch granules. Particularly if the desirable characteristics of the biopolymeric material are adversely affected by heat induced or oxidative damage, lower temperatures need to be employed for the dehydration process but this results in long drying times. It is sometimes necessary to employ a plurality of drying techniques to reduce the drying period and/or to achieve desirably low moisture contents but this, in turn, gives rise to considerable cost increases due to factors such as the amount of energy used. The energy consumption of such procedures is accordingly a significant factor in the overall production cost of biopolymers.
There is a recognized need for a simple, effective method for drying biopolymer materials which is environmentally friendly and economical in terms of energy consumption and cost. The method of dehydration should also be one that does not adversely affect the desirable characteristics of the biopolymeric material.
Both US-A-2004/0210046 and US-A-2008/0230050 describe a method for the physical treatment of starch (derivatives) using densified gases in which the starch is essentially treated at a process temperature above the critical temperature of the gas used and, in particular, between 31 and 180° C., and at process pressures between 5 and 80 MPa (50 and 800 bar) for at least one minute, the density of the densified gas being greater than 180 kg/m3. The starches thus treated are indicated to have reduced contents of water and lipids, and enhanced swelling and gelatinization behaviour.